Field of the Invention
The invention concerns a support for waveguides, which conduct mechanical waves. More particularly, the invention concerns a support for use in position sensors operating on the basis of the principle of the transit time of mechanical waves. The invention also concerns a method for producing such a support.
Position sensors operating in accordance with the principle of transit time measurements of mechanical waves in a metal waveguide generally have an elongate body of non-magnetic or plastic material constituting a housing for accommodating and supporting the mechanical waveguide. As a rule, a waveguide for conducting mechanical waves is made of a thin-walled tube, a wire or a strip. The waveguide may also serve as an electrical conductor.
Based on the Wiedemann effect, an exciter current pulse is fed into a waveguide and is superimposed with an external magnetic field. The external magnetic field is directed to the waveguide, in particular in a lateral direction. The current pulse thus produces a longitudinal or torsional pulse. The torsional and/or longitudinal pulse produced by the superimposition of the two magnetic fields is propagated approximately at a speed of about 2,500 to 6,000 m/s from the location at which it originates in both directions along the waveguide.
A detector detects the wave pulse at a reference position, which is usually at one end of the waveguide. The time period between the triggering of the exciter current pulse and the reception of the wave pulse after a transit thereof along the waveguide measures the distance of a displaceable positioning magnet from the detector.
A typical measuring arrangement which is based on the principle of transit time of mechanical waves is disclosed in U.S. Pat. No. 3,898,555.
The waveguide may be supported within the housing and is thus protected from the effects of external mechanical disturbances. Mechanical disturbances such as structure-borne noise, shock, or vibration may result in interference or noise signals in the position sensor arrangement. Those interference or noise signals are superimposed with the useful signal and cannot be distinguished therefrom. This results in incorrect measurements and the position measuring system cannot operate reliably.
In a number of position measuring systems the waveguide is supported by individual, spaced-apart support elements. The support elements are usually made from plastic material and have the shape of disks or conical elements. Such support elements are however expensive to produce and have to be fitted onto the waveguide by a threading procedure, which is a complicated and time-consuming operation.
Support elements of that kind are disclosed in the German patent document DE 33 43 310. A magnetostrictive tube is supported at suitable spacings by means of support rings. These support rings have a relatively narrow central opening through which the tube passes with only a small clearance. The support rings also define annular shoulders supporting portions of a protective tube surrounding the magnetostrictive tube.
A similar support configuration is described in the above-mentioned U.S. Pat. No. 3,898,555. The support has a plurality of rings arranged along the waveguide. Due to the particular design of this support configuration, it is difficult to handle the support elements.
An ideal support arrangement optimally protects the waveguide from external disturbances while damping or attenuating the torsional or longitudinal pulses, which are weak anyway, as little as possible.
A support arrangement having a woven, braided or plaited assembly with glass fibers, metal fibers, or polymer fibers and which is coated with a silicone rubber enclosure is disclosed in PCT-WO 96/35923. The waveguide is pulled into a hollow, double-layered cylindrical structure. The cylindrical structure prevents a lateral movement of the waveguide and protects the waveguide from the effects of vibration or external disturbances. The support configuration does not constrict the waveguide to such an extent that the mechanical waves, which are weak in any case, are damped or attenuated.
The above-described arrangement is highly temperature resistant and, compared to prior arrangements, reduces production costs for the manufacture and assembly of the waveguide support. Nonetheless, it suffers from a number of serious disadvantages, which have an adverse effect on the performance of the position measuring system with which it is used.
Due to the relatively high density of the material which makes up the woven or plaited glass fiber structure and the silicone rubber disposed thereon, vibrations and oscillations, caused by external excitation effects, may be transmitted to the waveguide, which can result in an interference. That effect is even further intensified by the high density of the external silicone layer.
A further disadvantage of this arrangement is that the tubular woven or plaited structure can be easily stretched, deformed, or upset, thereby changing the geometrical dimensions thereof and in particular changing its diameter. It is therefore not possible to guide and support the waveguide uniformly within the support arrangement, which however is an important requirement. The relatively hard material of the glass fibers of the inner layer can, in the event of external vibrations or oscillations, knock against the waveguide and cause interference signals.
The tubular support structure designed in the above-described manner has only little dimensional stability, due to the flexible material from which it is made. The woven or plaited assembly described in the above-mentioned document has a tubular woven or plaited material with a silicone coating thereon and is therefore highly flexible and not suited for mechanically supporting a waveguide.